1. Field of the Invention
The present invention relates to a wideband capable directional coupler.
2. Description of the Related Art
Directional couplers are used for detecting the levels of transmission/reception signals in transmission/reception circuits of wireless communication apparatuses such as cellular phones and wireless LAN communication apparatuses.
A directional coupler configured as follows is known as a conventional directional coupler. The directional coupler has an input port, an output port, a coupling port, a terminal port, a main line, and a subline. The main line has a first end connected to the input port and a second end connected to the output port. The subline has a first end connected to the coupling port and a second end connected to the terminal port. The main line and the subline are configured to be electromagnetically coupled to each other. The terminal port is grounded via a terminator having a resistance of 50Ω, for example. The input port receives a high frequency signal, and the output port outputs the same. The coupling port outputs a coupling signal having a power that depends on the power of the high frequency signal received at the input port.
Major parameters indicating the characteristics of directional couplers include insertion loss, coupling, isolation, directivity, and return loss at the coupling port. Definitions of these parameters will now be described. First, assume that the input port receives a high frequency signal of power P1. In this case, let P2 be the power of the signal output from the output port, P3 be the power of the signal output from the coupling port, and P4 be the power of the signal output from the terminal port. Further, assuming that the coupling port receives a high frequency signal of power P5, let P6 be the power of the signal reflected at the coupling port. Further, let IL represent insertion loss, C represent coupling, I represent isolation, D represent directivity, and RL represent return loss at the coupling port. These parameters are defined by the following equations.IL=10 log(P2/P1)[dB]C=10 log(P3/P1)[dB]I=10 log(P3/P2)[dB]D=10 log(P4/P3)[dB]RL=10 log(P6/P5)[dB]
The coupling of the conventional directional coupler increases with increasing frequency of the high frequency signal received at the input port, and thus has a non-flat frequency response. The conventional directional coupler therefore suffers from the problem of not being wideband capable. Where coupling is denoted as −c (dB), an increase in coupling means a decrease in the value of c.
U.S. Patent Application Publication Nos. 2012/0161897 A1 and 2012/0319797 A1 disclose directional couplers aiming to resolve the aforementioned problem. U.S. Patent Application Publication No. 2012/0161897 A1 discloses a directional coupler including first to fourth terminals, a main line connecting the first terminal and the second terminal, a subline provided between the third terminal and the fourth terminal, and a low-pass filter provided between the third terminal and the subline.
U.S. Patent Application Publication No. 2012/0319797 A1 discloses a directional coupler including first to fourth terminals, a main line connecting the first terminal and the second terminal, a first subline connected to the third terminal, a second subline connected to the fourth terminal, and a low-pass filter provided between the first subline and the second subline.
U.S. Patent Application Publication Nos. 2012/0161897 A1 and 2012/0319797 A1 each further disclose a directional coupler including first to fourth terminals, a main line connecting the first terminal and the second terminal, a subline provided between the third terminal and the fourth terminal, a first low-pass filter provided between the third terminal and the subline, and a second low-pass filter provided between the fourth terminal and the subline. The first low-pass filter is composed of a first inductor provided between the third terminal and the subline, and a first capacitor provided between the ground and the connection point between the subline and the first inductor. The second low-pass filter is composed of a second inductor provided between the fourth terminal and the subline, and a second capacitor provided between the ground and the connection point between the subline and the second inductor. The two U.S. publications each further disclose a directional coupler including two terminators, one between the first capacitor and the ground, the other between the second capacitor and the ground.
It is demanded of directional couplers for use in wireless communication apparatuses that signal reflection at the coupling port be reduced where the coupling port is connected with a signal source having an output impedance equal to the resistance (e.g., 50Ω) of the terminator connected to the terminal port. More specifically, it is demanded of the directional couplers that, where the return loss at the coupling port is denoted as −r (dB), the value of r be of sufficient magnitude in the service frequency bands of the directional couplers. An example of the cases where the coupling port is connected with the aforementioned signal source is where two directional couplers are connected in tandem for use. In such a case, the respective coupling ports of the two directional couplers are connected to each other.
Neither of U.S. Patent Application Publication Nos. 2012/0161897 A1 and 2012/0319797 A1 gives any consideration to reducing signal reflection at the coupling port where the coupling port is connected with a signal source having an output impedance equal to the resistance of the terminator connected to the terminal port. Further, for a directional coupler including a low-pass filter such as that disclosed in each of the above two U.S. publications, it is difficult to reduce signal reflection at the coupling port by simply adjusting the inductance of the inductor constituting the low-pass filter and the capacitance of the capacitor constituting the low-pass filter.
As previously mentioned, U.S. Patent Application Publication Nos. 2012/0161897 A1 and 2012/0319797 A1 each disclose a directional coupler including the first and second low-pass filters and two terminators, one of the two terminators being provided between the first capacitor of the first low-pass filter and the ground, and the other between the second capacitor of the second low-pass filter and the ground. The need for the two low-pass filters and the two terminators disadvantageously increases the size of the directional coupler.